A combined cycle power plant (“CCPP”) includes a gas turbine, a heat recovery steam generator (“HRSG”) and a steam turbine. The gas turbine includes a turbine configured to produce a rotational power output in response to an expansion of exhaust gases. The HRSG is configured to receive the exhaust gases from the gas turbine and generate steam from heat of the exhaust gases. The steam turbine is configured to rotate in response to an expansion of the steam from the HRSG.
The steam turbine is configured to operate with acceptable thermal stresses when a difference between a temperature of the steam entering the steam turbine and a temperature of the metal components of steam turbine is less than a predetermined temperature threshold. The temperature difference can exceed the predetermined threshold, during a startup of the steam turbine with the gas turbine operating at full load. Accordingly, the CCPP has a startup period of time within which the gas turbine load is gradually increased in order to gradually increase the temperatures of the steam and steam turbine, such that the temperature difference between the steam and steam turbine does not exceed the threshold.
Accordingly, it is desirable to provide a power generation system having an exhaust attemperating device and a method of controlling a temperature of exhaust gases to reduce thermal stress of a steam turbine and reduce a startup time, emissions and fuel consumption of the power generation system.